A very interesting book is currently up for auction on eBay. The Warden's Manual by S. (for Samuel) Evelyn Thomas looks to be jam-packed with interesting detail. It's a book I have not seen before, but I came across the author's name whilst researching at Hackney's archives. He was a very active member of the ARP and wrote a number of books about the service including "A Practical Guide To A.R.P.".
News about interesting insignia, ARP related info and period photos that turn up.